The use of a plurality of receivers and/or antemae in a diversity scheme are generally based either on selecting the antenna delivering the strongest received signal strength or having the largest signal to noise ratio. The decoder is switched from one signal to another on various bases, and only decodes the selected signal. The conventional basis for changing receivers and/or antemnas are:
A. Selective Combining: uses a plurality of receivers and antenna and simply selects strongest of the received signals.
B. Switched Combining: uses a single receiver with a plurality of antemae and only switches from the current signal (antenna) when the strength of the current signal is reduced below a preset level.
C. Maximum-Ratio Combining: uses a plurality of receivers and antemae, weights the signals, co-phases and combines them to provide a combined signal which is the sum of the instantaneous signal-to-noise ratios of the individual branches.
D. Equal-Gain Combining: uses a plurality of receivers, co-phase and combines the signals by bringing all phases to a common point and combines them to provide a signal that is the sum of the instantaneous fading envelopes from each receiver.
A type following system B above which minimizes the equipment and still provides a corrected signal (uses a single receiver between a pair of antemas) is described for example in U.S. Pat. No. 4,696,058 issued Sept. 22, 1987 to Tachita et al.
U.S. Pat. No. 4,328,582 issued May 4, 1982 to Battail et al discloses a system using parallel channels with binary signaling and weighs the binary values by a metric based on the bit error rate to provide a binary value with a maximum likelihood of being correct by calculating the error probability for each signal and applying the optimum decision rule to obtain the most likely binary value.
U.S. Pat. No. 4,404,674 issued Sept. 13, 1983 to Rhodes is somewhat similar to the system described in the U.S. patent to Battail et al discussed above, but applies reliability numbers based on hard-detected code symbols and bases the decoding decision on these numbers to obtain the most reliable signal.